Python Basics — Error Handling

Error Handling

Errors are a natural part of programming. Python provides a powerful and elegant exception-handling system that lets your programs deal with problems gracefully — instead of crashing.

What You’ll Learn

Topic	Description
Exceptions Overview	Built-in exceptions, hierarchy, reading tracebacks
try/except/else/finally	The complete error-handling toolkit
Raising Exceptions	When and how to signal errors
Custom Exceptions	Creating your own exception classes
Debugging Techniques	Tools and strategies for finding bugs
Practice Exercises	5 hands-on challenges

Quick Preview

Before Error Handling

# ❌ Program crashes instantly
age = int(input("Enter age: "))  # User types "abc" → ValueError!
print(f"You are {age}")

After Error Handling

# ✅ Program handles bad input gracefully
try:
    age = int(input("Enter age: "))
except ValueError:
    print("Please enter a valid number!")
else:
    print(f"You are {age}")

Error Types at a Glance

Type	Example	When It Happens
SyntaxError	if True print("hi")	Code is malformed (caught before running)
TypeError	"3" + 5	Wrong type used in operation
ValueError	int("abc")	Right type, wrong value
KeyError	d["missing"]	Dictionary key doesn’t exist
IndexError	[1,2,3][5]	List index out of range
FileNotFoundError	open("nope.txt")	File doesn’t exist
ZeroDivisionError	10 / 0	Division by zero
AttributeError	"hi".append("!")	Object lacks the attribute/method
NameError	print(undefined_var)	Variable not defined
ImportError	import nonexistent	Module not found

Why Error Handling Matters

• User experience — show helpful messages, not tracebacks
• Data protection — save progress before exiting on errors
• Program resilience — recover from problems and keep running
• Debugging — log errors for later investigation
• Professional code — expected behavior for all inputs, not just happy paths

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Start Learning: Exceptions Overview →